


Evil Twin

by TheWhiteLily



Series: Season Four Premiere Flashfic [8]
Category: Sherlock (TV)
Genre: But Eurus was a worse big sister, Episode: s04e02 The Lying Detective, Gen, Holmes Family, Kids through to adults, Lily's grand unified theory of everything, Mental Instability, Mycroft was a rubbish big brother, POV Mycroft Holmes, Pre-Canon, SNAFU, Spoilers, Written prior to The Final Problem
Language: English
Status: Completed
Published: 2017-01-12
Updated: 2017-01-13
Packaged: 2018-09-17 01:26:45
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 2
Words: 4,210
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/9298064
Author URL: https://archiveofourown.org/users/TheWhiteLily/pseuds/TheWhiteLily
Summary: When he was seventeen, my brother deleted all memory of his sisters.  Both of them.  All he remembers about them now is this: it’s never twins.





	1. Chapter 1

**Author's Note:**

> I don't know if there's any more information on Eurus than was in the episode, but while trying to confirm the spelling I came across what looked like an interview with the creators about her, and closed it down quicksmart. It bugs me I'm not sure what it says, but not enough to risk spoilers. :)

When he was seventeen, Sherlock Holmes deleted all memory of his sisters. Both of them. All he remembers about them now is this: it’s never twins.

* * *

“Myc?”

Mycroft read to the end of the paragraph, and then the next one, before looking up at his younger brother.

He’d taken off his pirate hat to come inside—Mummy’s insistence—and was fiddling with it, turning it in his hands.

“Yes, Sherlock?”

“Eurus made it so that Georgie doesn’t like me anymore. She says I should never have any friends, because only family is worthy. She says, if I try to talk to him again, she’ll—”

Mycroft sighed, and closed his book. He supposed that wasn’t entirely wrong. Although potentially overstating the case where family were concerned.

“Sherlock,” he said, with the utmost patience. “Georgie doesn’t like you because you deduced that his mother was having an affair, aloud, in front of his father. Now his parents are spending all night, every night shouting and throwing things. If you try to talk to him again, he’ll probably punch you in the nose. You have to learn when to _keep things to yourself_.”

He gave the last few words a meaningful emphasis, because this was dangerous. Sherlock was dancing on a knife-edge, and if he started telling other people about Eurus, Mycroft wouldn’t be able to do anything to protect him.

* * *

It had started with a story.

Mycroft had liked to tell it; liked the idea of the wind that scoured the earth clean of the unworthy; liked the idea that it could be personified and called forth by someone who knew how.

Mummy had pointed out the name when he was nine and she was helping him with the translation of the more difficult words in _Odyssey_.

She’d always liked it, she said, but then again, she’d always thought of the wind as being a woman.

Sherlock had been only two then, but Mycroft had taken over responsibility for his brother and sister’s bedtime stories some time ago. Father had always done the stories, _before_.  He'd been good at them _—_ passionate and engagingly animated over the fantastical tales he preferred _—_ and Mummy could become distressed at that memory when she tried to step into his shoes.

In hindsight, perhaps Mycroft should have known better, but really. Sherlock might have been a little slow, but he wasn’t exactly a baby anymore, and they were all of them bored with _Winnie-the-Pooh._

* * *

Mycroft came awake, utterly still, at a sound from the hallway outside.

“Myc?” the whisper came again, tentative and unsure. “Are you awake?”

The small form was standing in his doorway again, as it did many nights, half hidden behind the frame, half behind his mop of dark curls.

“What is it _now,_ Sherlock?” sighed Mycroft. He’d been up late, involved in a fascinating text setting out the state of international relations in the lead up to the Great War, and some of the minor government officials who’d had the most impact on the gathering forces.

“Eurus says she killed Redbeard, Myc.”

“Sherlock… I know you can say my name properly now.”

“Mycroft,” Sherlock corrected himself quickly. “She said…” His voice faltered, and then strengthened again. “She said I was getting attached. And that he was unworthy. So the East Wind took him.”

Mycroft put one hand over his face, rubbing the sleep from his eyes. He was really starting to regret telling Sherlock that story.

“Redbeard was in renal failure,” he said. Surely any six year old could understand that, even one crushed by… _grief_. Really, one would think he would be old enough to be past that sort of thing. “You went with him when he had to to be put down yesterday.”

“Eurus _said_ she gave him something longacting and slow, a _poison_! She said she _killed—_ ”

“Sherlock, Sherrinford doesn’t have a secret twin,” snapped Mycroft, slamming his hand down on the bed beside his body. “There is no one called Eurus in our family. She was only ever a bedtime story. You just had a childish bad dream. Now go back to bed before the East Wind takes _you_ , too, for bothering me in the middle of the night.”

He rolled over, pulling his blanket firmly over his shoulder, and ignored the sensation of watching eyes from the doorway.

* * *

Sherrinford had always been Sherlock’s favourite. She’d always been everyone’s favourite, once people got over the disconcerting shock of her mismatched eyes.

At first glance, though, they tended to be every bit as repelled by her unusual abnormality as they were entranced by his brother’s more subtle sectoral heterochromia.

But while Sherlock was too boisterous, too enthusiastically obnoxious, and thus made the gushing words of praise for his beauty turn quickly against him, Sherrinford was quite the opposite.

She wasn’t exceptionally pretty, but she had the knack of fitting in, of seeing people and sliding effortlessly into the role that best met their expectations.  She was the sweet little girl in a pretty party dress to their parents’ friends, the conscientious high achiever to her teachers, the tomboy in red wellies scrambling over the rocks to crew a pretend pirate vessel to Sherlock. At home in any company, such a congenial girl quickly found her disconcerting eyes forgiven and forgotten.

Mycroft saw her for who she was: less of an idiot than most, gifted in an area which he’d often found bemusingly opaque, but still frustratingly slow in all the ways that mattered.

His own eyes were unaffected by any genetic abnormality.  They were a plain, watery blue—a drab colour that meant no one ever asked for a closer look—and they were set too narrowly above a nose too long.

No one, including himself, had discovered any evidence that Mycroft was possessed of a personality at all. He didn’t care to endear himself to others like Sherrinford did, nor was he so unaware as to alienate them like Sherlock did. If Mycroft spoke again after the pleasantries were concluded, it was because he had something worth saying, and someone worth saying it to. If not, he simply faded beside his brighter-burning siblings—the oldest and cleverest, watching from the wall as others danced, the ever-present spectre at the feast.

It seemed rather the more sensible way to do it, instead of wasting all that energy on other _people_.

* * *

“Myc! Wake up! Eurus says—”

“She doesn't have a twin, Sherlock.”

“But—”

“Sherlock. Eurus does not exist. She’s in your head. Do you understand? She was only ever a story. You _need_ to hold on to that.”

* * *

Mycroft remembered events around Sherrinford’s birth quite clearly. He’d been four and perhaps slightly more sure of himself at that tender age than had been warranted, but only slightly. It was the first time he’d become aware of Father’s… issues.

His new sister—and yes, there had definitely only ever been one—had been born with sea-blue eyes, the colour of Mycroft’s, although Mummy warned him that might change. It was only after she was six months old that one eye had started to darken, while in the other seemed to bleach an even paler sky-blue in contrast.

Father didn’t respond well to the burden of stress placed on the family by an extra child. There were long stretches of time where he couldn’t be convinced to move from his bed. Those were better than the bouts where he didn’t sleep for days, embarking on grand renovation plans that involved hammering and drilling all hours of the day and night. Or when he ranted and raved along the lines of unrealistic conspiracy theories, flinched from the blows of invisible assassins in the air, and conducted fierce inspections of the whole family, demanding to know when they’d been replaced with imposters.

Mostly, once he’d sorted out what was real, he was fine.

The family managed to work around him for a further two and a half years as he lost his job, threw the medication he was supposed to be taking back in their faces, and increasingly ignored their patient explanations about the difference between fantasy and reality.

And then one morning, when he was six, Mycroft had come down the stairs to find the living room had been pulled apart, the couches slashed and the stuffing spread everywhere, the wallpaper torn down in great strips. Father stood on a chair holding a knife in one hand and throwing books down from the shelf with the other, muttering under his breath about people wanting to hurt his family and no one being who they seemed.

Mummy was curled in a corner of the room, arms covering her head from the hail of books, and crying.

After a moment to assess the situation, Mycroft turned, as yet unseen, and slipped back into the hallway to pick up the phone and call the police. If Father wasn’t capable of exerting enough intellectual power over his illness to control it anymore, then someone else was going to have to do it for him.

It was several weeks after Father been taken away that Mycroft realised Mummy wasn’t just sick and worn out from the stress and the crying and the obnoxiously cheerful face she put on when visiting Father. She actually _was_ sick. She was going to have another baby.

There was another chance at the lottery now for Father’s flawed genetics; another roll of the dice for a child who might inherit his instability.

* * *

“What is it this time, Sherlock?”

“Eurus is going to _hurt_ my—”

“There is no Eurus, Sherlock! Go to _sleep_!”

* * *

They were a happyish family now, Mycroft thought, as best as he could judge these things.

Mummy kept everyone in line: Sherlock and Sherrinford played endless imaginary games—which was likely where the obsession with Sherrinford having a twin had started—and their stepfather existed in a blissful, semi-sentient daze. And of course Mycroft had his books.

If only Sherlock could get a handle on his paranoid obsession with the personification of a children’s story before anyone _noticed_.

He wouldn’t remember what it had been like, visiting Father in the institution. The only father figure he remembered was the dull, good-natured goldfish Mummy had replaced him with, even if he had long ago deduced the man was not a biological relation.

Ugh. How could he be?

But Mycroft remembered Father, both before and after his confinement. Sherrinford would remember, too; she’d been five when Father had finally decided that he didn’t want to live in the institution anymore, bypassed the laughably mundane security restrictions on his padded cell, then made his way out to the pharmacy and out of life itself.

Mycroft and Sherrinford, while never having been particularly close, shared a tacit understanding of the necessity of helping Sherlock learn to rise above his condition.

It would definitely upset Mummy if Sherlock ended up like Father, too.

* * *

Eventually Sherlock’s paranoid midnight visits stopped, and Mycroft slept easier, not only because of the lack of interruption.

Mental illness might run in the family, but with Sherlock’s level of intelligence and the right training in controlling his emotional responses, he should be able to be high-functioning enough to cover for it. Father had done that for years, after all, and he’d been barely smarter than Mummy.

Mycroft continued to remind Sherlock of the risks of his wild, uncontrolled behaviour whenever it cropped up and risked exposing the instability underneath.

He had to hope that would be enough.

**Notes for the Chapter:**

> Apologies to _Winnie-the-Pooh_ , which no one could possibly grow bored of.


	2. Chapter 2

When Sherlock was thirteen, Sherrinford moved on to Cambridge, robbing him of his favourite playmate. He made no attempt to find another companion among his schoolmates and, being Sherlock, he probably wouldn’t have been capable of sustaining a friendship with anyone who didn’t have to put up with him anyway.

It quickly became clear he was becoming dangerously bored, investing all his time in chasing up criminal cases in the paper. Mycroft began bringing him little problems from work; easy things, well within his capabilities.

Or so he’d thought.

“It’s not twins, Sherlock,” he said tiredly, reaching to pull the offered file back towards himself. “Gerald Phelps doesn’t have any siblings.”

Sherlock set his jaw. “If _I_  was a twin, _I_ would—”

Mycroft was up on his feet now, angry. Somehow Sherlock had always been capable of drawing that out of him, in a way no one else had.

“Checking a birth record is a matter of moments, Sherlock! As is confirming every other mark a human being makes on the world, _and_ verifying the recollections of their families. There is no way of hiding the existence of an entire person from infancy, and no reason to attempt such a venture in the _absurdly_ unlikely expectation that one or both of the newborns will need an alibi. _It. Is. Never. Twins!_ ”

Mycroft unclenched his fists, breathing hard, and brushed himself down before retaking his seat.

Sherlock blinked several times quickly, the adolescent sneer settling into uncertainty.

 _At last_.

“Oh,” he said. “No, of course it isn’t.”

He did eventually work out that the man was being blackmailed for bigamy, but given Mycroft had already alerted Lady Smallwood to the issue in her department before he’d left to visit home, Sherlock’s less-than-optimal time to solve the problem didn’t really matter.

* * *

When she’d finished her schooling with a double first, Sherrinford joined Mycroft in government work, drawing every eye and moving, chameleon-like, through the levels of bureaucracy.

She wasn’t as clever as Mycroft, but she was better at being noticed for her cleverness—which was both a good thing and bad, although she was an expert at judging the line where being noticed would become being noticed too much. And she had a true gift for undercover fieldwork, for blending in and becoming a seamless part of the picture around her, even more so when she had access to a range of coloured contacts to disguise her distinctive eyes.

As a treat on school holidays, Mycroft began exposing Sherlock to more and more of the boring legwork cases that stole time from Mycroft’s day. Once he’d finished his A levels, Mycroft even arranged for an internship. The keen young agent he'd handpicked to direct and protect his brother while he skipped around troubleshooting had been a good choice—and he’d been strangely unbothered by Sherlock’s antisocial outbursts.

Sherlock’s covert glances and enthusiastic efforts to impress him made it clear that he had more on his mind than the security of the nation, but he was nonetheless becoming skilled enough to be an actual help. He hadn’t mentioned twins in years at this point—and at least Sherlock’s uncharacteristic deference made it easier for the lad to keep him in line.

All three of them, even Sherlock, were far above the capabilities of the mindless cretins who appeared to populate the rest of the world—and Mycroft far above the other two, even if _people_ had always been Sherrinford’s special area of expertise. But Mycroft continued to pass with by far the least notice of the Holmes siblings.  He’d never minded amassing favours and quiet respect in the background. Real power would always lie in being underestimated, as would freedom. The kingmakers he would one day join in shaping the future knew who he was already; those who were too stupid to respect his time didn’t know enough to seek him out.

So it was that when documents began going missing—when an important mission was hopelessly compromised by a double agent known only as Grendel, documents stolen and an entire team turned up dead—that it was to Sherrinford that most eyes turned to deduce the identity of the culprit.

But it was Mycroft who noticed a pattern to the five missing documents that Sherrinford hadn’t seen. Sherlock was safely at Cambridge reading Chemistry: fortunate, because the situation was far too dangerous to involve him and besides, the young agent who’d proved so adept at handling him had been among the lives lost. So, while his showier sister ran distraction, Mycroft quietly, subtly, put a word in a few key ears to arrange his own sting in the background.

He did arrange to be there at the moment it all came together, which was fortunate, because if it hadn’t been for him, she may well have talked her way out of it.

“Sherrinford,” he said mildly, walking toward her in a warehouse suddenly flooded with bright light and men in riot gear, swinging his umbrella nonchalantly. He hid his surprise well enough that he had no doubt his colleagues would miss it, but his sister had always been able to read people’s reactions, even his.

She smiled, visibly relaxing. She could see Mycroft’s shock, of course, but could obviously also see the certainty in his eyes and desisted in her attempts to explain away her presence in the honeytrap.

“Oh, clever big brother,” she said. “Always the _clever_ big brother. I should have known this was your hand at work.”

“But _why_ , Sherrinford?” he questioned, genuinely confused. He’d thought her smarter than this. “It’s such a short-term game you’ve played here, and for what?”

His sister tilted her head on one side, her posture and demeanour changing utterly in an instant.

“It doesn’t understand,” she said, her intonation strange. “It _still_ doesn’t understand. Then again, it was always too clever for its own good. Too easy to fool a _clever_ mind, if it can’t imagine you might be cleverer than it.”

It _was_  his sister. It wasn’t Sherrinford.

Mycroft crossed his hands on the handle of his umbrella and leaned on it, hoping he didn’t look like he was holding himself up as a hundred instances of young Sherlock coming to him for help to fight the monster in his head and being denied flooded through his mind.

_Truth, truth, truth._

The East Wind, coming to pluck up the unworthy.

Oh, _Sherlock_. Always getting attached. Always looking for the complex explanation, when the reality was so much more simple.

It was no longer Sherlock to whom he regretted telling that story.

Sherlock had never known Father. He never knew to hide the impossible things he believed until Mycroft had taught him.

But Sherrinford had always known. She’d covered for it as long as she could.

He met the stranger in a familiar face’s bicolour gaze, and gave an insincere smile as he depressed the hidden button on the umbrella under his hands, activating the distress signal. They were going to need more backup, because this woman was _not_ going to escape due punishment.

A Holmes gone rogue was a serious security concern, and what she’d done to Sherlock over the years changed absolutely nothing.

“You must be Eurus,” he said, and sent her a repressive look at her rolled eyes. “I think you’ll find, sister, that the East Wind can come for you, too.”

* * *

“What are you doing?” demanded Sherlock, seventeen and outraged and having no business on the street outside the operation, but apparently emotional enough use his gifts to follow up on the death of his bodyguard. “Where are you taking her?”

“Eurus is dangerous,” Mycroft told him blandly, glancing at the woman standing beside him between two burly agents. “She’s committed treason and murdered at least six people to get to Agent Trevor. She needs to be locked away.”

“What, all this time, and suddenly you believe in secret twins?” scoffed Sherlock, sharing a look of disgust with Sherrinford that provided only a thin cover over his upset. “I gave up trying to convince you of that _years_  ago; Sherrinford’s been helping me look for Eurus without you. But you’ve got the wrong one, Mycroft. That’s Sherrinford there, can’t _you_ tell them apart?”

“It was _never twins, Sherlock_!” snapped Mycroft.

Sherlock set his face into the same mulish expression he’d worn every time they’d had this conversation, and Mycroft made himself take a deep, slow breath.

“Sherrinford _is_ Eurus,” he said, as patiently as he could. “And Grendel. And who knows how many other people. They’re smart enough to cover for it. They worked together to conceal it. But you liked Sherrinford, and you couldn’t imagine her body might be host to more than just one consciousness, that she might be helping it. You were the _only_ one who ever saw Euros! Caring is not an advantage, Sherlock. How was I supposed to know she was more than a figment of your imagination, when you wouldn’t see they were _the same person_!”

“I,” said Sherlock, looking suddenly lost. “Sherrinford?” he asked her in desperation.

“Don’t worry about us, Sherlock,” she told him reassuringly, and reached out to touch his face, her voice serious. “Big Brother’s found us, but he can’t hold us. The East Wind will still protect you. She will still come, for anyone unworthy, anyone your silly, ordinary heart tries to latch on to outside the family. _Anyone._ ”

Sherlock reared back from her, obviously seeing the stranger in her gaze as Mycroft had, even though her expression hadn’t changed. He stared at her in horrified recognition.

“We _will_ contain her, Sherlock,” Mycroft said, and motioned to her guards to move her away. “I’ll look after it personally. She won’t get out. And I’ll… explain things to Mummy.”

Sherrinford looked over her shoulder at them at that and laughed, insanely, and then changed again and began making cheerful conversation with the agents beside her. Sherlock finally came unfrozen and backed away a few steps, still unable to tear his eyes from her, before he turned and ran, disappearing into the night.

Mycroft sighed, and let him go. He needed to ensure their sister was secured.

* * *

The next time Mycroft tried to talk to Sherlock about Sherrinford, tried to—yes there really was no other word for it—tried to _apologise_ , Sherlock had no idea who he meant.

“Eurus?” he said. “That old story about the East Wind, Mycroft? I’m not a _child_ anymore. _”_

He seemed to be serious.

Sherlock had claimed to have deleted things before, but never… never a _person_. Never the most formative, most joyous moments of half his childhood. Nor the most frightening, secret terror that had kept him up at nights. The only friend he’d ever had—and the one who’d made sure it stayed that way.

He seemed colder. Less considered. More immature. Less _sentimental_. More intense. Less… Sherlock.

Mycroft probed, carefully asking about childhood memories, about what had happened to Redbeard and Georgie, as short-lived as his relationship with both had been. About his original plans to be a pirate and even, in a desperate last ditch effort, suggested that the solution to the puzzle he’d brought for him might be a previously unknown identical twin.

“Don’t be ridiculous, Mycroft,” snapped Sherlock, without flinching. “It’s never twins.”

* * *

Years later, Eurus escaped. There was a manhunt with Mycroft directing forces and surveillance in a massive chess match across half of southern England as a gameboard, trying to find her before she reached Sherlock—or the silver-haired Detective Inspector he’d been solving puzzles for recently.

The CCTV around Sherlock’s flat in Montague Street had been out for almost ten minutes before someone picked up the fault, and when he arrived to check on his brother, Mycroft burst through the door at a flat run.

Sherlock sneered at his panting concern; obviously hadn’t the faintest clue of the true identity of the potential client he’d been talking to. She hadn’t stayed long. Laughed, apparently, when he’d referred to “Geoffrey” Lestrade as being a useless, bumbling idiot, and left not long after, to be picked up around the corner and returned placidly to her cell.

Of course Sherlock had been high, but when wasn’t he these days?

Still, it was disconcerting to think that he could have sat face to face and conversed with the sister he’d once cared for so deeply and still not remember her.

* * *

When Doctor John Watson arrived in Sherlock’s life in an explosion of gunpowder and compact loyalty, Mycroft set a grade three active surveillance watch on them both. And he tripled the frequency of his random check-ins on Sherrinford’s cell, the yellow note on his refrigerator never showing a date more than a week away.

Sherlock was getting attached again. Quickly. Deeply. And without regard for safety.

He didn’t even remember their sister. Or her evil twin. Didn’t remember the danger. Mycroft probed again occasionally to check, making snide little remarks about Redbeard, and piracy, whenever he saw a flash of the brother Sherlock had been, but there was still not the merest flicker of recognition.

Moriarty was a further consideration, his worrying obsession with Sherlock too familiar and too close to the bone for Mycroft’s comfort—but Sherrinford _was_ still safely locked away, incommunicado and without any way of engaging the services of a master criminal to be her cat's paw.  Mycroft increased the surveillance on her another level: she was a legitimate security concern.  The fact that he was Sherlock’s brother changed absolutely nothing.

Still, as long as she _was_ secure, Sherlock’s friend—Sherlock’s battered, ordinary heart—would be safe from her.

And after the way he’d let his brother down the first time, Mycroft was going to make sure it stayed that way.

**Notes for the Chapter:**

> Thanks for reading. If you enjoyed--or were intrigued, or just want to discuss theories--I'd love to hear from you!


End file.
